This invention relates to a joined ceramic-metal composite substrate having a copper sheet directly joined to a ceramic substrate and a method for the production thereof.
In recent years, DBC (Direct Bonded Copper) substrates have been developed for ceramic circuit boards which are intended for substrates for ceramic circuit boards which are intended for substrates in transistor modules, for example. They are obtained by disposing a copper sheet at a stated position on a ceramic substrate and directly joining the sheet and the substrate by heating. The heating is carried out at a temperature not exceeding the melting point of copper (1,083.degree. C.) and exceeding the eutectic point of copper and oxygen (1,065 .degree. C.). The copper sheet is later formed into conductor circuits.
In the DBC substrate, the ceramic substrates are heavilY warped or deformed during the application of heat. Consequently, gas emanating from the ceramic substrates or copper sheets is entrapped in the interfaces between the ceramic substrates and copper sheets, and the copper sheets sustain fine blisters in portions thereof, or portions of insufficient union are produced in the composites. When these blisters grow to a size exceeding a certain level, they create a problem in that the DBC substrates will not permit silicon semiconductor chips to be easily mounted thereon and consequently the yield of the products is poor. There is another problem in that even when the chips can be mounted, the chips have poor adhesion and suffer from inferior thermal conductivity.
Jochym (USP 4,409,278Z) has disclosed a directlV joined metal-substrate composite which has venting channels between the contiguous surfaces of the metal sheet and the substrate. This construction evidently prevents the occurrence of blisters in the interface. However, it was found by the present inventors that even if this kind of construction is used in such a composite substrate, it does not often provide highlY satisfactory thermal conductivity. This is because such construction permits the etching liquid and liquid material such as soldering flux to enter into the grooves in the mounting process, and such liquid prevents the interface between the copper sheet and the ceramic substrate from having good adhesion. It is a problem when such substrate has poor thermal conductivity. This is because thermal conductivity is important for these substrates to effectively dissipate heat from silicon chips.